Over 15 million tons of sand per year is mined from areas that contain various iron species. Sand mined for glass and semiconductor production must be freed from various impurities, especially iron. Glass and semiconductor manufacturers have strict iron specifications that most sand mines cannot meet without using iron removal procedures. Iron species such as limonite and hematite are a few examples of the impurities typically found in sand bogs. Both are species of Fe2O3. The iron compounds that normally stain sand particles or form conglomerates are one of the most difficult impurities to remove using conventional processes. At present, the mineral process industry typically utilizes attrition scrubbers to separate iron and sand under caustic conditions. The process consists of three pH adjustments where the influent pH is typically low (3–5) and must by adjusted to pH of 10 or greater. Then the scrubber effluent must be acidified to pH 2–3 to separate the tails and iron. After the separation, the water is neutralized to pH 7. However, this process often does not remove enough iron to increase the utility of the sand in glass and semiconductor production.
U.S. Pat. No. 4,405,588 discloses a method of removing iron impurities adhering to silica sand particles, comprising grinding the ore, washing and desliming the ground ore to remove the major part of the clay-type binder, attrition scrubbing the deslimed particles to release further amounts of binder, washing and desliming to remove the binder, drying and heating the ore particles, treating the hot particles with a chemical agent suitable for converting the iron impurities into water soluble iron compounds, attrition scrubbing the hot suspension of chemically treated particles to release the iron stains, and washing with cold water and desliming to recover the purified particles. The chemical agents disclosed are sulfuric acid, hydrochloric acid and caustic soda.
U.S. Pat. No. 5,334,364 discloses a process for purifying silica sand consisting of subjecting the sand to centrifugal rotation under dry conditions while simultaneously agitating the particles, and then eliminating the impurities from the sand particle mixture.